Controlled clearance piston gauges are used as primary pressure standards. See “Intercomparison of Hydraulic Pressure Measurements to 28 MPa Using a Single-Piston Gauge in the Controlled-Clearance, Reentrant, and Simple Configurations”, K. Jain et al., Rev. Sci. Instum., Vol. 63, No. 5, May 1992, pp. 3127-3135 incorporated herein by this reference. See also “The Controlled-Clearance Principle”, Donald H. Newhall, Industrial and Engineering Chemistry, Vol. 49, No. 12, December 1957, pp. 1993-1995 also incorporated herein by this reference.
A controlled clearance piston gauge includes a piston surrounded by a jacket which can be pressurized to reduce the space between the piston and the cylinder bore until a zero clearance condition is achieved at a jacket pressure called the zero clearance pressure (Pz). The pressure measured by a controlled clearance piston gauge is a function of many parameters including the masses loaded onto the piston and the effective area of the piston which itself is a function of the zero clearance pressure.
The zero clearance pressure, in turn, is typically determined by measuring the rate at which the piston sinks into the cylinder (also called the fall rate) at different jacket pressures. This measurement may involve the use of a capacitance-type displacement or inductive-type transducer. Also, in some cases, particularly for pneumatic piston-cylinder assemblies with highly precise geometries, the sink rates are much too slow to measure the sink rate accurately enough for the purpose of estimating the zero clearance pressure.